Food Colourants

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Caramelization is defined as the thermal degradation of sugars leading to the formation of
volatiles (caramel aroma) and brown-colored products (caramel colors).
The process is acid or base catalyzed and generally requires temperatures > 120oC at 9<pH<3.

How is caramelization different from Maillard reaction? 

Caramelization occurs in food, when food surfaces are heated strongly, e.g. the baking and roasting processes, the processing of foods with high sugar content such as jams and certain fruit juices, or in wine production.

Chemistry of the reaction

The generation of flavours and colours in thermally induced caramelization requires that sugars, normally monosaccharide structures, to first undergo intramolecular rearrangements. The reaction causes the release of H+. Thus the pH of the solution undergoing caramelization falls with time, eventually into the slightly acidic region of pH 4-5.

Caramelization occurs in a sequence of 6 steps:

The initial enolization reaction is of particular importance because it initiates the subsequent chain of events. These reactions give rise to aliphatic sugar degradation products which can react further to produce oxygen heterocyclic and carbocyclic compounds via aldol condensation.
Step 1:  Enolization or de Bruijn van 
Echenstein Rearrangement
Step 2:  Dehydration or b -Elimination
Step 3:  Dicarboxylic Cleaving
Step 4:  Retro-Aldol Reaction
Step 5:  Aldol Condensation
Step 6:  Radical Reaction

The key intermediates of the thermal caramelisation are the osuloses.
They are a -dicarbonyl compounds such as 3-deoxyhexosulose. These not only lead to the formation of caramel colour but also give rise to the important volatile products which are typical of caramel flavour.

Which reducing sugar below has the highest rate of color development
during caramelization browning?
Maltose  Lactose  Fructose  Glucose  Xylose

The rate of color development of reducing sugars decreases as the pH decreases; in the case of sucrose, however, the situation is somewhat more complicated owing to the acid-catalyzed sucrose hydrolysis, which produce fructose and glucose.

Some typical components of caramel flavour are shown in Fig. 2.
There are 4 general categories:  furans, furanones, pyrones, and carbocyclics.

The relative proportions of these compounds can be influenced by the temperature and whether the conditions are acidic or basic. While the thermally and/or acid-induced dehydration and cyclisation reactions dominate, there is an increased tendency for cleavage of the carbon chain of the sugar under basic conditions. Thus, depending on the reaction conditions, one finds derivatives of furan, such as hydroxymethylfurfural (HMF) and hydroxyacetylfuran (HAF), furanones such as hydroxydimethylfuranone (HDF), dihydroxydimethylfuranone (DDF) and the pyranones "maltol" from disaccharides and "hydroxymaltol" from monosaccharides.

It has been proposed that a structure of the type CH3-CH=CH-CH(OH)-C=O is responsible for the characteristic caramel flavour (i.e. this moiety is part of the structure of HDF).

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